There are forklift trucks equipped with a hydraulic driving device referred to as an HST (Hydro Static Transmission) between a power source, i.e. an engine, and driven wheels. The HST includes a variable displacement travel hydraulic pump, driven by the engine, and a variable displacement hydraulic motor, driven by hydraulic fluid discharged by the travel hydraulic pump, in a main hydraulic circuit, which is a closed circuit, and transmits the driving power of the hydraulic motor to the driven wheels to enable a vehicle to travel.
A work vehicle, such as a forklift truck, is operated to perform switch-back, in which its forward-backward lever is operated from a backward position to a forward position while the vehicle is traveling backward, or the forward-backward lever is operated from the forward position to the backward position while the vehicle is traveling forward, so that the vehicle slows down until it stops traveling backward or forward and, immediately after that, the vehicle travels forward or backward with increasing speed (see Patent Literature 1, for example).